Scanning microscope with feedback means to maintain a constant target current

ABSTRACT

The invention includes a scanning mirror microscope having a feedback control loop between target and cathode to stabilize the target current. It is then possible to obtain a quantitative plot of the target voltage by scanning an electron beam over the target surface. The invention also includes a scanning electron microscope having a feedback control loop between the target and the output of a velocity analyzer of secondary electrons emitted from the target to obtain the same quantitative plot. In either case, examination tests on semiconductor integrated circuits are made possible.

MEANS TO MAINTAIN A CONSTANT TARGET CURRENT 1 4 Claims, 2 Drawing Figs.

US. Cl. 250/49.5 Int. Cl. 1101i 37/26, GOln 23/04 Field of Search 250/4958, 49.59, 49.51

. Unlted States Patent 1111 3,55 35 [72] Inventor John P. W. Flemming [56] References Cited Harlow, Essex, England UNITED STATES PATENTS g $.9 3:3 a 2,418,228 4/1947 Hillier 250/4958 E 4 z f Jan-23 1971 3,219,817 11/1965 Mollenstedt.. 250/4951 3,351,755 11/1967 Hasler 250/4958 920 5mm! 3,404,271 10/1968 Lehovec m1. 250/4951 New York, N.Y. Primary Examiner-James W. Lawrence a corporation of Delaware Assistant Examiner-C. E. Church [32] Priority Oct. 25, 1967 Att0rneysC. Cornell Remsen,Jr.,WalterJ. Baum, PaulW. [33] G t B it i l-lemminger, Percy P. Lantzy and Thomas E. Kristofferson [31] 48468/67 ABSTRACT: The invention includes a scanning mirror I [54] SCANNING MICROSCOPE WITH FEEDBACK microscope having a feedback control loop between target and cathode to stabilize the target current. It is then possible to obtain a quantitative plot of the target voltage by scanning 1 an electron beam over the target surface.

The invention also includes a scanning electron microscope having a feedback control loop between the target and the output of a velocity analyzer of secondary electrons emitted from the target to obtain the same quantitative plot.

In either case, examination tests on semiconductor integrated circuits are made possible.

jjscANNmo-Mrcitoscon: WITH rssouxcx MEANS 'ro MAINTAIN A CONSTANT 'rxnoar BACKGROUND or THE "mvarmou energies lyingwithin a'certainnarrowly-defined-range.

well known that a qualitative .plot of the voltagevaria- 't i on(expressed as afltnction ofposition) across the-surface of a specimen may be obtained by making the specimen the tar get in a scanning mirror microscope. The plot canbe obtained 'byrscannin'g the electron beam of the microscope over the surface'cf the target and recording the resulting variation in targetcurrent. Another method 'of'obtaining the same .type of qualitative plot is to make the specimen the target, in a scanning "electron microscope. In this method, the voltage variation across the target can be derived from a measure of the'eurrent output from a velocity analyzeras an el'ectron beam; is scanned over the surface of the target analyzer is placed to receive the, secondaryele'ctrdns' emitted by" the target under the influenceof the electron beam. However in eithergmethod, it' is not a straightforward matter to derive a quantitative plot of the voltage'variation because the voltage variation -in each method is a nonlinear function of thecurrent detected. Nevertheless, in many appli cations, particularly in the examinationof semiconductor int'egrated circuits, it is'desirablejto obtain aquantitative voltage plot; and this-invention improves scanningmirrormicroscopes and 'scanningelectron microscopes and enables such a plotto beobtained-by means of a'feedbackcontrol loop. r s

suMMARYor ruemveurioiv s applied to scanning mirror microscopes, the invention provides such a microscope that includes a feedback control loo p which inrelation to a target inserted into the microscope and the electron beam'cathode of the microscope, can-be actuated by the output currentfrom the targetso t'hatthe potentialdifference between the cathodeandthe target can *be' controlled to. reduce 'fluctuationszin said output as the electron beam scansythesurface of theta-rget 1 Asj applied "to: scanning electron microscopes, 'the "invention provides. such a microscope-including a velocity analyzer and a feedback-control loop. The loop in relation to a targe t:in-'

'serte'dintothe microscope and the velocity analyzer, can be actuated by the output from the velocity analyzer-so that the potential of the targetcan be controlled. with respect to the electron energy level accepted by the velocity analyzer in order to reduce fluctuations in said output as the electron beam scansthe surface of the target.

The features of the 'invention'will, be-evid'ent from *the following description when considered in connectio'n'withthe accompanying drawings.

,[BRlEF uesciurrloworTHBDRAWINGS i lnthe drawings, which-areto beregarded-asumerel y illustrativIez" l1 1 l lG. 1 is a diagram of a scanning mirrormicroscope incorporating afeedbackcontrol loop, and v l lGr 2gisa diagram of a scanning electron microscope-incor- 'porating a feedback control loop.

,With'reference to FIG. 1, a scanning mirror microscope is shownfincluding a cathode Migwhichis-the source of an electron; rbeam, an, electron beam forming arrangement .1 1 Complete with-facility'for scanningthe beam, and in the path of: the electron beam,a target 12%having a terminal connected at its-base. The target -l 2is connected by terminal 13', both througha resistorR toground and through a, capacitor 1 4, to

theijnputof an amplifier-T5.v The output'of amplifier IS'is connectedthrough another capacitor PM =to the'cathode- 1 andalso islconnected directly'to. an output terminal 173,11 he connection between the target 12 and the cathode I0 is through the two'capacitors 14 and 16 and the amplifier 15. The connection forms the feedback path'by which the target current is stabilized. The. quantitative voltage variation across target 12 may be obtained at the output terminal. a,

The operation of the loop will be evident from the following analysis. The following'notation is used:" F

V, the potential of that part stantaneously intercepts the prima H the microscope, V the potential of the cathode, and L a V the potential of the target terminal 13,

where these potentials are referredto ground potential.

electron beam scans the target betweentwo adjacent points across,-there will 'be' achange in potential v in the voltage 'm- 1 ln order to prove the effectiveness of the feedback loop in stabilizing target current, it is necessary to show that theeffect of the feedback loop is to reproduce the variation vffasa variation in the voltage at the output terminal 17 (noted'a For this purpose, the conductance 3" of the 'rnirror microscope is defined by: 1'

This definition makes g is a constant; and to justify the use of g as a constant, it will be necessary to show the effect of the feedback loop is to reduce v compared to v The relationship (1) shows that the voltage at the output terminal approximates the required voltage, provided that A and AgR are'large compared to l. The relationship (2) shows that the expression AgR is the loop gain of the feedback control loop. in other 'words'for an accurate output, the amplifier must have a large gain and the loop .gain of the feedback control'loop must also be large. The relationship (3) shows that the feedback loop has the effect of reducing the fluctuation magnitude of v Further, it shows that if the amplifier gain and loop gain are large, fluctuations of v are small compared to the variations .inpotential across the surface of the target v The relationship also indicates that .g may be assumed to be constant,

With reference to FIG. 2', a scanning electron microscope is, shown including an electron beam generator 20, complete with facility-for focusing andscanning. An electron beam 21 from the generator 20wis incident upon a target 22 which, in

the-absence of a feedback loop, is at a fixed potential with target which in-" ctron of respect to the cathode potential of the electron beam generator 20. Secondary electrons then are emitted from target 22. A proportion 23 of these secondary electrons having the appropriate energy are intercepted by a velocity analyzer 24 which accepts electrons within an energy range fixed with respect to the cathode potential of the electron beam generator 20. The output of the velocity analyzer 24 is fed to an amplifier 25 whose output voltage is coupled through a capacitor 26 to the target 22. The output from the amplifier also is connected to an output terminal 27 from which may be obtained the required information about the variation of the potential I across the surface of the target 22.

The feedback control loop between the velocity analyzer 24 and target 22 operates in a manner analogous to the loop in the scanning mirror microscope described above. In the scanning electron microscope, the potential difference between the primary electron beam cathode and the target is so large, the energy distribution of secondary electrons (expressed as a function of energy with respect to the potential of that part of the target from which they are emitted) is nearly independent of small variations of the surface potential of the target. However if the energy distribution is expressed as a function of energy with respect to some fixed potential, this fact implies that the secondary electron energy distribution is dependent upon such variations of potential.,-Therefore, the electron current intercepted by the velocity analyzer is a function of the potential of the target surface. This point may be contrasted with the fact that in the mirror microscope, the target current is a function of the potential with respect to the cathode of the target surface. Hence by analogy, in the case of the scanning electron microscope, instead of a measure of the target current variations fed to the input of the feedback amplifier, the input to the amplifier is a measure of the derived from the interception of electrons by the velocity analyzer;

and instead of the voltage output from the amplifier being fed to the primary electron beam cathode, it is fed to the target.

It is to be understood that the foregoing description of specific examples of this invention is made by way of example only and is not to be considered as a limitation on its scope.

I claim:

1. An electronbombardment examination device comprising; a scanning mirrormicrosc'ope. including means to scan an electron beam acro a target anda cathode; and a feedback.

control loop adapte to Be actuated by the output current from said target, said loop being'connected to control the potential difference between said cathode and said target in a manner to reduce fluctuations in said output current as said electron beam scans over the surface of said target.

2. The invention as defined in claim 1, wherein said feedback control loop includes an amplifier responsive to said output current for controlling the potential difference between said target and said cathode inversely therewith. v

3. An electron bombardment examination device comprising: a scanning electron microscope including means to scan an electron beam across a target; a velocity analyzer; and a feedback control loop adapted to be actuated by the output 

1. An electron bombardment examination device comprising; a scanning mirror microscope including means to scan an electron beam across a target and a cathode; and a feedback control loop adapted to be actuated by the output current from said target, said loop being connected to control the potential difference between said cathode and said target in a manner to reduce fluctuations in said output current as said electron beam scans over the surface of said target.
 2. The invention as defined in claim 1, wherein said feedback control loop includes an amplifier responsive to said output current for controlling the potential difference between said target and said cathode inversely therewith.
 3. An electron bombardment examination device comprising: a scanning electron microscope including means to scan an electron beam across a target; a velocitY analyzer; and a feedback control loop adapted to be actuated by the output from said velocity analyzer to control the potential of said target according to the electron energy level accepted by said velocity analyzer and in such a manner as to reduce fluctuations in said output as the electron beam scans the surface of the target.
 4. The invention as defined in claim 3, wherein said feedback control loop includes an amplifier responsive to the output of said velocity analyzer for controlling the potential of said target inversely therewith, said velocity analyzer being positioned to intercept secondary electrons from said target. 